As existing bridges and their roadway decks age they deteriorate due to the effects of repeated traffic loads, environment, loss of paint, and use of deicing chemicals and therefore need to be maintained with ever-increasing care, and in many cases must be replaced to ensure safety. Some older bridges were never designed to handle modern heavy truck traffic and are therefore structurally deficient. As populations grow, so does the volume of traffic. The consequence is that there is increasing pressure in the United States, and abroad, to modify and strengthen existing bridge structures and to develop more durable, less expensive, lower maintenance, lighter weight, and more easily assembled bridge structures for the future. There are also environmental concerns associated with application and removal of protective paint systems on steel structures which must be taken into consideration.
The typical bridge has a superstructure and foundation system by which a bridge roadway is supported at a desired elevation relative to adjacent terrain. As the moving loads of traffic traverse the bridge, the deck and the superstructure, eventually deteriorate. In some cases, the superstructure and foundations were never designed to support today's heavy trucks. A key factor in obtaining improved bridge structures therefore is to reduce the weight of the bridge deck without sacrificing strength, rigidity, durability, and the ability to cope with unusually heavy loads, accidents and the like. Traditional steel and concrete bridge decks are heavy and are subject to deterioration. Steel superstructures and reinforcing steel in concrete tend to rust and therefore require expensive anti-corrosion measures, inspection and/or painting. Steel orthotropic decks, while considered to be light in weight, are usually heavier than aluminum decks, require extensive welding and are fatigue sensitive. They are also quite flexible in the transverse direction, i.e., across the principal direction of traffic flow, which leads to wearing surface failures, and may be more expensive than aluminum.
Bridges typically consist of a superstructure and a substructure. The superstructure includes the deck and any members which support the deck that are oriented in a generally horizontal configuration. Bridge superstructures often include steel beams. When these beams run parallel to the length of the bridge (called the longitudinal direction of the bridge) they are referred to as girders or sometimes as stringers. Steel beams running transversely to the direction of traffic sometimes are also provided as part of the bridge superstructure.
Bridge decks are typically made of concrete with steel reinforcing bars, although some decks are made of steel plate with ribs on the underside running in the longitudinal direction. These steel decks are referred to as steel orthotropic decks because they have significantly different structural properties in the longitudinal and transverse directions. They are more costly than concrete decks but typically weigh less. One problem associated with steel decks is that the wearing layer typically applied on top of the upper steel, to provide a skid-resistant surface for traffic, often fails prematurely.
Concrete decks are typically cast in place at the bridge site. This requires a significant expenditure of time and labor to prepare the formwork and falsework needed to cast the concrete and to allow the concrete to cure. Steel and aluminum decks are fabricated off-site under controlled conditions and with more efficient labor in shops. Metal deck fabrication typically includes longitudinal and transverse splices between smaller parts that make up the deck. However, there are practical limits to the size of fabricated pieces that can be shipped. Therefore, steel and aluminum decks may also require longitudinal and transverse splices at the bridge site.
Serious consideration is therefore being given to the use of light-weight, corrosion resistant, easily-handled, aluminum deck structures. To reduce costs while ensuring high quality, attention has focused lately on forming the bridge deck in modular fashion, i.e., with initial construction being carried out in a shop or factory with the resulting modular elements being quickly and relatively inexpensively assembled in the field. Prefabricating "deck panels" or "deck slabs" from selected numbers of constituent elements also gives the bridge designer additional freedom in selecting the dimensions and form of the resulting bridge deck.
Examples of known bridge deck structures which variously address such needs include U.S. Pat. No. 4,709,435 to Stemler et al, U.S. Pat. No. 4,912,795 to Johnson, U.S. Pat. No. 5,033,147 to Svensson, and U.S. Pat. No. 5,414,885 to Berlin et al. These and other comparable prior art references teach different ways of forming bridge deck structures from component elements including extruded aluminum elements having hollow cross-sections, and the use of a wearing surface on an upper surface of the bridge deck.
The joints between adjacent elongate elements in the prior art, e.g., Svensson, are subject to flexing open and closed under loading, which can result in potential reflective cracking of the wearing layer. The joints between adjacent elongate elements in the present invention are welded, and so will not tend to produce cracks in the wearing layer when the deck is loaded. The Svensson elongate elements are clamped to the bridge girders. This method of attachment cannot be relied upon to transmit shear between the girder and the deck, since only an unquantified friction is available to transmit this shear. Thus, the benefits of composite action of the girders and deck cannot be realized. Also, since it is the practice of bridge engineers to assume that clamped joints will likely freeze up due to the accumulation of dirt or oxides, the deck and girder must also be designed as if shear were transmitted between them. This means that the bridge must be investigated for two conditions and the worst effects of the two used for the design. The Svensson type of structure also requires that holes be drilled in the bridge girders and that shims be driven between the deck and girders to anchor the deck at every joint between the elongate elements. This may be time-consuming and expensive.
There is, however, a continuing need for improvements which would increase the capacity of the bridge, reduce the cost (including the cost of assembling the structure from prefabricated modular elements), tolerate occasional overloads by overweight vehicles or caused by accidents or the like, and meet all applicable governmental standards and professional codes. The present invention is intended to meet such demands.
The present invention comprises an aluminum bridge deck. While somewhat similar to steel orthotropic decks in that they weigh less than concrete or filled grating, aluminum decks weigh even less than steel decks. Also, as will be explained further below, this invention teaches how aluminum decks can be made with essentially isotropic, rather than orthotropic, properties. With a continuous bottom flange and a continuous top flange, as in the preferred embodiment per FIGS. 10 and 12, for example, loads can be effectively resisted by two paths, i.e., in bending longitudinally and transversely to the length of the elongate elements. This is more structurally efficient than providing only one load path to resist loads. It is also redundant, and offers greater structural reliability. The net result is an essentially isotropic deck. Structural strength in this deck structure, in both shear and bending, is thus provided both longitudinally and transversely to the direction of traffic.
Other cross-sectional forms of the basic element from which such aluminum decks are formed provide varying combinations of advantages.